Method of forming a grating in a waveguide

ABSTRACT

A refractive-index grating fabricated in an optical fiber having a multilayer coating and a method for making refractive-index patterns such as gratings in optical fibers such that the mechanical properties of the original fiber are preserved. The patterns are written into the optical fiber by partially stripping away the outer coating of the fiber, exposing the core of the fiber through the remainder of the coating with an actinic radiation to form the pattern in the photosensitive core of the fiber, followed by recoating the fiber in the stripped area to provide protection of the newly formed pattern from corruption and to preserve the mechanical properties of the fiber.

FIELD OF THE INVENTION

[0001] The present invention relates to optical media and, moreparticularly, to the fabrication of refractive index patterns such asgratings within optical fibers. A major objective of the invention is toprovide for faster fabrication of refractive-index gratings in opticalfibers, with less cost, and without significantly reducing the fiber'smechanical strength.

BACKGROUND OF THE INVENTION

[0002] Gratings in optical fibers are important structures for opticalcommunications. For example, communications systems using wavelengthdivision multiplexing require gratings to separate the various signalstraveling through the optical fibers. Fiber gratings are also used tomake sensors. Most fiber gratings are presently fabricated by exposingthe core of the fiber to a UV light, having a wavelength around 240 nm,that causes a change in the refractive-index of the fiber's core.However, because the outer polymer coating of most optical fibers is nottransparent to 240 nm light, the fiber's outer polymer coating must beremoved before exposing the fiber core to the UV light to form thegrating. The fiber must then be recoated to prevent damage to the glassfiber and to preserve the mechanical strength of the fiber. Thisrecoating must be done in a timely manner because exposing the surfaceof a silica fiber to humidity and dirt can permanently weaken the fiber,and the mechanical strength of the once-exposed fiber can remaindecreased even after the silica core is subsequently recovered with acoating material. The choice of recoating material is limited by therequirement that the recoating material must adhere well to silica andmay need to form a hermetic seal to the fiber surface. Additionally,removing the fiber's polymer coating before UV light exposure andsubsequently recoating the fiber with a polymer after the UV lightexposure is time consuming and expensive.

[0003] Recently, new fiber coatings that are transparent to UV light at257 nm have been used to coat optical fibers. These new fiber coatingsmake it possible to fabricate fiber gratings without first having tostrip the fiber of its coating. However, these fibers with their specialcoatings have several disadvantages. The transparency of the coating toUV light makes the fibers sensitive to the environment, sinceundesirable UV light from the environment can now reach thephotosensitive fiber core, producing excess optical loss and, in extremecases, even erasing the fabricated grating. Additionally, these coatingsare especially soft and sticky, and can accumulate dust. This dust canadversely affect grating fabrication if the dust absorbs UV light duringthe fabrication process. Moreover, the polymer coating can also becomedamaged by excess heat, which can also distort the fiber grating in thefiber core.

[0004] An alternative approach for writing gratings in fibers withoutremoving the fiber coating uses the sensitivity of the fiber core tolight in the near-UV region of the spectrum, having a wavelength ofapproximately 330 nm. An advantage to using near-UV light instead ofmid-UV light is that the polymer-coating of standard opticalcommunication fiber (such as Corning SMF 28, a product of CorningIncorporated, Corning, N.Y.) is somewhat transparent to near-UV light,but is not transparent to mid-UV light having a wavelength ofapproximately 240 nm. Because standard polymer coatings are transparentto light in this near-UV wavelength region, it becomes possible tofabricate gratings through standard coatings without the use of aspecial polymer coating. Standard fiber coatings also provide protectionto the photosensitive fiber core from mid-UV light having wavelengths inthe region of the spectrum where the fiber core has its highestphotosensitivity, so that the problem of induced loss and gratingerasure by UV light from the environment is reduced. However the problemof degradation of the polymer coating surface from dust and otherenvironmental contaminants remains, because such dust can absorb UVlight when the grating is written and distort the light pattern thatforms the grating in the fiber core. Special measures to protect thephase mask from contamination with dust and possible exhaust from thepolymer during UV exposure may also be required.

[0005] What has been needed, and heretofore unavailable is a faster,lower cost method for writing refractive index gratings into opticalfiber that avoids the problems of damage or contamination of the coatingand subsequent deterioration of the optical path needed to write thegrating into the fiber core region. The resulting fiber must have highimmunity to erasure or solarization and must retain a significantfraction of its original mechanical strength. The present inventionfills this need.

SUMMARY OF THE INVENTION

[0006] Briefly and in general terms the present invention solves theproblems of protecting the photosensitive fiber core from undesirableenvironmental UV exposure and shielding the optical polymer surface fromdegradation. This is accomplished by using a fiber having multiplecoating layers. This multicoated fiber contains an inner coating layerthat is mostly transparent to the writing light, and a “protective”coating layer over the inner “optical” layer to provide mechanicalprotection for the inner layer from dust and other contaminants, andoptical protection of the photosensitive fiber core from undesiredphoto-darkening and solarization. The protective layer is easy to removewithout significantly disturbing the inner layer. This removal of theprotective layer can be accomplished by either mechanical or chemicalmeans. The protective layer is removed primarily in the region where theoptical writing exposure is executed, for example only around the regionof the fiber where the grating will be written. Removal of theprotective layer should be done immediately before writing with UV lightso the outer surface of the inner layer will have no time to degrade oraccumulate dust. After writing with UV light the protective layer andany other outer layers are then reformed in the regions where they wereremoved.

[0007] One key advantage of the new technique as compared to thestandard stripping and recoating technique is that the silica glass ofthe fiber itself is never exposed to the environment so that the fiberretains a greater mechanical strength. Exposing the surface of a silicafiber to humidity and dirt can permanently weaken the fiber, and themechanical strength of the once-exposed fiber can remain decreased evenafter the silica core is subsequently recovered with a coating material.Another advantage of the new technique is that the protective polymerdoes not necessarily have to adhere well to silica; the recoatingprocess does not require hermetic sealing of the outermost coating tothe fiber surface. The less stringent requirements for stripping andrecoating simplify these processes so that time and expense can besaved.

[0008] One embodiment of the invention comprises a waveguide forprocessing with actinic radiation. The waveguide, in the form of anoptical fiber, has a coating containing at least an inner layer and aseparate protection layer. The inner layer may be at least half astransmissive as absorptive for actinic radiation, where the actinicradiation may have a wavelength longer than 220 nm although otherwavelengths of actinic radiation may also be used. The core of thewaveguide is sensitive to actinic radiation. The protective layer of thewaveguide is removable either mechanically or chemically and can beremoved without removing the inner layer.

[0009] The waveguide of this type allows for a method of processing thewaveguide so as to affect the sensitive core. The method consists ofremoving the protective portion of the coating followed by exposure ofthe core to the actinic radiation through the remainder of the coating.This exposure can be uniform or non uniform along the waveguide. Onepurpose of exposing the waveguide and its sensitive core to the actinicradiation is to fabricate a grating structure within the waveguide. Manydifferent types of patterns or gratings may be produced within the coreof the waveguide. Examples of such gratings include Bragg gratings andlong-period gratings. The method can be used to produce long periodgratings covering a range of average periods in the range of 10 to 2000microns.

[0010] The method also includes the recoating of the waveguide with aprotective layer after its exposure to actinic radiation. Thisprotective layer should be at least half as absorptive as transmissivefor the actinic radiation. The protective layer is typically a polymer,which may be a polymethacrylate, silicone resin or any other appropriatematerial including, aliphatic polyacrylates, silesesquioxanes,alkyl-substituted silicones or vinyl ethers.

[0011] The actinic radiation is typically Ultra-Violet radiation in theregion 220-390 nm, although other wavelengths of actinic radiation mayalso be used. The method can also include preloading the waveguidebefore exposure to UV light with hydrogen (or its isotope deuterium) toincrease the photosensitivity of the core to the UV light.

[0012] Another embodiment of the invention includes a refractive indexgrating formed within the core region of an optical fiber by exposure ofthe fiber core to actinic radiation. The grating containing fiber has amechanical breaking strength of not less than 50% of the same fiberprior to processing the fiber to fabricate the grating. It isanticipated that the breaking strength could actually be 90% or more ofthe strength of the original fiber. The optical fiber used infabrication of the grating is the same as previously discussed, havingan inner layer and a separate protective layer, with the protectivelayer being removed prior to creation of the grating, then replaced witha new protective layer in the region containing the grating.

[0013] Other features and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, thefeatures of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic view of an embodiment of the presentinvention showing an optical fiber having a removable protective layerand a transparent inner coating layer.

[0015]FIG. 2 is a flow diagram, essentially in block form, of a processfor forming a fiber grating in the optical fiber of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] As shown in the exemplary drawings, the present invention relatesto optical media and, more particularly, to the fabrication ofrefractive index patterns such as gratings within optical fibers. Thepresent invention provides for faster, less costly fabrication ofrefractive-index gratings in optical fibers, without significantlyreducing the fiber's mechanical strength. The grating formed in such anoptical fiber can be of any pattern formable by exposure of aphotosensitive fiber core to actinic radiation to provide regions ofvarying refractivity withing the core of the fiber. For example, thegrating could be a Bragg grating or a long-period grating, two types ofgratings currently in general use.

[0017] One embodiment of an optical fiber incorporating the presentinvention is depicted in FIG. 1. The optical fiber 20 consists of a coreregion 21 surrounded by a cladding layer 22 that is itself surrounded byan outer coating. The core 21 of the fiber is doped to be opticallyreactive to certain wavelengths of an actinic radiation typicallyUltra-Violet light greater than 220 nm although other wavelengths ofactinic radiation may be used as needed to produce the effect desired inthe core 21 of the fiber 20. In some cases the core of the fiber can beloaded with hydrogen (or its isotope deuterium) to increase thephotosensitivity of the core to UV light. Typical loading conditions are4000 psi of H₂ at 30°-100° C. for 1-6 days.

[0018] The coating layer of the preferred embodiment is a multilayercoating having with at least two layers, a protective layer 24, and aninner layer 23 that is typically bonded to the fiber cladding layer 22.The inner coating layer 23 should be more transparent than absorptive ofthe actinic radiation used to write the grating into the fiber core 21.Since the actinic radiation used to form gratings in fiber optic coresis usually UV light having a wavelength longer than 220 nm, the innercoating layer 23 must transmit a greater percentage of the UV light inthis wavelength than the layer 23 absorbs. Moreover, it is preferablethat the layer 23 be more transparent than absorbing to near-UVradiation having a wavelength in the 275 nm-385 nm range. One source forthe actinic radiation may be light having a wavelength around 257 nmproduced by a frequency doubled Argon laser, although other sources oflight having the desired wavelengths are well know by those skilled inthe art.

[0019] The refraction index of the inner coating layer 22 may beselected to be greater than, less than or the same as the refractionindex of the adjacent cladding layer 22, depending on the requirementsof the grating pattern to be fabricated. In a preferred embodiment, theinner coating layer 23 may be formed from a polyacrylic or a siliconeresin, although other materials, such as aliphatic polyacrylates,silesesquioxanes, alkyl-substituted silicones or vinyl ethers may alsobe used.

[0020] The protective layer 24 protects the outer surface of inner layer23 from dust or other contaminants, and is preferably easy to removefrom the inner layer 23 without significant damage to the inner layer23. Ease of removal of the protective layer 24 may be enhanced byselecting a material for the protective layer 24 that has differentmechanical properties or different chemical properties than the innerlayer 23.

[0021] Inner layer 23 in turn protects the surface of the fiber glasscladding 22 from environmental factors when the protective layer 24 isremoved, such as humidity and dirt which can reduce the strength of thefiber. While inner layer 23 is transparent to the actinic radiation usedto write the grating into the fiber, the protective layer 24 is formedfrom a material that is absorptive or reflective of the spectrum ofradiation to which the fiber core 21 is photosensitive. The absorptiveor reflective property of the protective layer 24 protects the fibercore 21 from the effects of ambient environmental radiation when thefiber is used. This protection prevents corruption or erasure of thegrating by solarization. Moreover, since only a portion of theprotective layer 24 is removed during formation of the grating, asdepicted by removed area 28 in FIG. 1, the protective layer 24 preventscorruption or changes in areas of the fiber core 21 that are outside ofthe desired location of the grating.

[0022] Referring now to FIGS. 1 and 2, a method of forming a grating inan optical fiber incorporating the present invention will be described.Typically, a grating is formed by exposing a portion of the core layer21 of an optical fiber to actinic radiation having a wavelength to whichthe material forming the core layer 21 is photosensitive. The protectivelayer 24 is removed from the inner layer 23 in a selected region 28 ofthe optical fiber 20 where the grating is to be located. Removal can beeither complete or sufficient to a. Removing the protective layer 24 inregion 28 exposes the inner layer 23. Preferably, the protective layer24 is removed just prior to writing the grating so that the quality ofthe outer surface of inner layer 23 will be preserved by protectivelayer 24 during all preliminary stages of fiber processing and handlingincluding any hydrogen loading of the fiber core 21 or cladding layer22.

[0023] When the protective layer 24 has been stripped from region 28,actinic radiation 26 from light source 27 is directed towards theoptical fiber 20 through a mask 25 to form a periodic pattern in thefiber core 21. Since the protective layer 24 absorbs or reflects thewavelengths of the actinic radiation 26 produced by light source 27, theonly portion of the fiber core 21 that is exposed to the actinicradiation 26 is the area of the fiber core 21 that lies under the innerlayer 23 that is exposed in region 28. As previously discussed, theinner layer 23 is formed from a material that is more transparent thanabsorbent to the actinic radiation 26, thus ensuring that actinicradiation passing through the mask 25 is transmitted through the innerlayer 23 and into the core 21 and cladding 22 of the optical fiber 20.

[0024] After the grating is formed in the core 21 of the optical fiber20, the inner layer 23 is preferably recoated to reform the protectivelayer 24 in the exposed region 28 so that the previously exposed region28 has the same mechanical and light absorbing or reflecting propertiesas the adjacent unstripped regions of the fiber. One additional benefitof the preferred invention is that the recoating of exposed region 28 issimpler and easier to perform than prior art methods because thecladding layer 22 of the optical fiber 20 is already coated with theinner layer 23, and the material used to recoat and reform theprotective layer 24 in region 28 need only to adhere to the inner layer23 and not to the silica glass of the core 21 or cladding layer 22.

[0025] The fiber can be annealed with heat after writing the grating tostabilize the grating. Annealing can also be used to change theproperties of the coating, for example, increasing the coating'sabsorption in the UV range to protect the photosensitive fiber core orto increase the mechanical strength of the coating. In some cases,annealing may make recoating of the fiber unnecessary.

[0026] The following examples are provided to illustrate two possiblemethods for the forming of gratings with partial removal of theprotective layer 24. These examples are provided as illustrations onlyand are not intended to limit the scope of the disclosed invention.

EXAMPLE 1

[0027] An optical fiber having a core and a cladding layer is coatedwith UV transparent methylsilsesquioxane to an outer diameter of 170microns. The fiber, now comprising a core, cladding layer and thetransparent layer of methylsilsesquioxane is then coated with a layer ofUV absorbing polyvinyl to an outer diameter of approximately 800microns. After the coating is completed, the optical fiber may be loadedwith deuterium to increase the photosensitivity of the fiber core to UVlight.

[0028] A grating is formed in the optical fiber at a desired location bymechanically stripping the polyvinyl coating layer from the transparentmethylsilsesquioxane layer, and the fiber core is exposed to 257 nm UVlight from a frequency-doubled Argon laser through a phase mask. The UVlight passes through the fiber's transparent methylsilsesquioxane layerto reach the cladding and core. A Bragg grating having 3 dB reflectionat 1550 nm forms in the fiber's core after exposing the core through thetransparent methylsilsesquioxane layer for 5 minutes with 40 mW of 257nm continuous-wave actinic radiation. After exposure of the fiber to theactinic radiation, the length of the fiber optic containing the gratingis annealed for 2 days at 90° C. in an oven. When the portion of thefiber optic contain the grating has been suitably annealed, the strippedportion of the fiber optic is recoated with the protective polyvinylouter coating.

EXAMPLE 2

[0029] An optical fiber having a core that is co-doped with boron andgermanium and a cladding layer is coated with a UV transparent innerlayer formed from a silicone resin to an outer diameter of 190 microns.The coated optical fiber is then coated with a nitrocellulose-basedprotective layer to form an completed optical fiber with an outerdiameter of 250 microns. A grating is formed by dissolving a portion ofthe protective layer in acetone where the grating is to be located, andthen illuminating the exposed silicone resin layer through a phase maskwith near-UV actinic radiation having a wavelength of 334 nm from aUV-Argon laser. A Bragg grating with 10% reflection forms in the fibercore as a result of the near-UV exposure. The fiber is then annealed at200° C. for 2 minutes. The exposed silicone resin region of the fiber isthen recoated by dipping the exposed region of the fiber into anitrocellulose solution and subsequently evaporating the solvent.

[0030] From the foregoing, it will be appreciated that the disclosedoptical fiber with multilayer coating and the disclosed method forcreating refractive index patterns in the core of the optical fiberthrough the inner coating of the fiber provide a faster, lower costmethod for writing refractive gratings into optical fiber. It providesthese advantages without the incurring the problems of the prior artmethods of grating formation by preventing contamination of the coreregion with dust or allowing corruption or erasure of the grating byenvironmental light to which the core is photosensitive. The resultingoptical fiber, containing the refractive index pattern, retains a highimmunity from erasure or solarization and retains a significant part ofits original mechanical strength.

[0031] While particular forms of the invention have been illustrated anddescribed, it will be apparent that various modifications can be madewithout departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited, except asby the appended claims.

What is claimed is:
 1. A waveguide comprising: a core region sensitiveto an actinic radiation; and a coating having at least an inner layerand a protective layer where said inner layer is at least half astransmissive as absorptive for said actinic radiation.
 2. The waveguideof claim 1, wherein the actinic radiation has a wavelength longer than220 nm.
 3. The waveguide of claim 1, wherein the protective layer isless transmissive than absorptive of the actinic radiation.
 4. Thewaveguide of claim 1 wherein said protective layer is removable withoutremoving the inner layer.
 5. The waveguide of claim 4, wherein saidprotective layer is removable by mechanical means.
 6. The waveguide ofclaim 4, wherein said protective layer is removable by chemical means.7. The waveguide of claim 1, wherein said protective layer isdissolvable.
 8. The waveguide of claim 1, wherein said inner layer is apolymer.
 9. The waveguide of claim 8, wherein said inner layer ispolyacrylate.
 10. The waveguide of claim 8, wherein said inner layer isTeflon.
 11. The waveguide of claim 8, wherein said inner layer is asilicone resin.
 12. The waveguide of claim 1, wherein the core region isformed in an optical fiber.
 13. The waveguide of claim 12, wherein thecore region further comprises a core surrounded by a cladding layer. 14.A method of forming a grating in an optical waveguide having a core anda coating, the coating having at least an inner layer and a protectivelayer, comprising the steps of: removing the protective layer of thecoating in a selected portion of the waveguide; forming a pattern withinthe core of the optical waveguide by illuminating the waveguide withactinic radiation.
 15. The method of claim 14, wherein the step ofilluminating the waveguide includes illuminating the waveguide in amanner which is non-uniform along the length of the waveguide.
 16. Themethod of claim 14, wherein the step of illuminating the waveguideincludes illuminating the waveguide in a manner which is uniform alongthe length of the waveguide.
 17. The method of claim 14, furthercomprising the steps of: providing a phase mask; illuminating thewaveguide by passing the actinic radiation through the phase mask toform a pattern of regions having varying refraction indexes in the coreof the waveguide.
 18. The method of claim 14, further comprising thestep of recoating the waveguide with a protective layer after formingthe pattern in the core.
 19. The method of claim 18, wherein thewaveguide is recoated with a polymer.
 20. The method of claim 18,wherein the waveguide is recoated with polymethacrylate.
 21. The methodof claim 19 wherein the waveguide is recoated with a polymer selectedfrom the group consisting of aliphatic polyacrylates, silesesquioxanes,alkyl-substituted silicones and vinyl ethers.
 22. The method of claim14, wherein the step of illuminating includes illuminating the waveguidewith directing Ultra-Violet radiation having a wavelength in a range of220 nm-390 nm.
 23. The method of claim 14, wherein the step ofilluminating the waveguide includes illuminating the waveguide withUltra-Violet radiation having a wavelength of less than 275 nm.
 24. Themethod of claim 14, further comprising the step of loading the waveguidewith a material selected from the group consisting of Hydrogen (H₂) andits isotopes prior to the step of forming the pattern.
 25. The method ofclaim 24, wherein the isotope of Hydrogen selected to load the waveguideprior to the step of forming the pattern is Deuterium (D₂).
 26. A fiberoptic refractive index grating formed in an optical fiber, comprising:an optical fiber having a core region surrounded by a cladding layer,the core region being sensitive to actinic radiation; a coatingsurrounding the cladding layer, the coating having at least an innerlayer and a protective layer, the inner layer being at least partiallytransparent to actinic radiation and the protective layer beingsubstantially non-transparent to actinic radiation; a pattern of varyingrefractive indexes formed in a selected portion of the core region ofthe optical fiber.
 27. The fiber optic refractive index grating of claim26, wherein the portion of the core region having the pattern has amechanical breaking strength greater than 50% of the mechanical breakingstrength of the remainder of the optical fiber.
 28. The fiber opticrefractive index grating of claim 26, wherein the portion of the coreregion having the pattern has a mechanical breaking strength at least90% of the mechanical breaking strength of the remainder of the opticalfiber.
 29. The refractive index grating of claim 26, wherein the gratingis a Bragg grating.
 30. The refractive index grating of claim 26,wherein the grating is a long period grating.
 31. The refractive indexgrating of claim 30, wherein the grating has an average grating periodwithin the range of 10 microns to 2000 microns.
 32. The refractive indexgrating of claim 30, wherein the refractive index of the inner layer isless than the refractive index of the cladding layer.